When are docker images rebuilt?

Hello everyone,
this is a question mostly out of curiosity.
I was trying to get a new commit to the 2.3 Snapshot on github because it fixes an issue I had opened against the openhab2-addons repo.
Now, I run openhab in a docker container and so I simply tried to get the newest docker image, so that the fix would be included.
I noticed, however, that the latest image on hub.docker.com is over 2 days old.
Then I looked at the way this docker image is being built and it turns out, that the docker file pulls the distribution zip from cloudbees in order to build the image.
Now my question is: What determines when the official docker image is rebuilt?
And also: why does it take days to update when the image is called “Snapshot” and as such would imply that it is kept as up-to-date as possible with the github repo?